Can't access custom properties of a linked characters proxy rig
I decided to use a workaround. Being able to use the custom properties would be a slick option as far as UI / user friendliness goes.
But for now I've decided to create one bone per face shape and treat them like sliders where translating the bone in Y axis would drive the face shape.
This works in linked files.
If anyone wants more details about this setup I can write up something.